Immunization of marmosets with marmoset (interspecies) platelet preparations leads to severe thrombocytopenia and ultimate death of the animal. The presence of antiplatelet antibodies, elevated numbers of megakaryocytes in the bone marrow and the absence of splenomegaly in these animals offers a remarkable parallel to the clinical problem of post- transfusion purpura, a specific disorder falling within the 'autoimmune' syndrome of idiopathic thrombocytopenic purpura (ITP). The ability to induce this phenomenon in the marmoset by injections of platelets from closely related animals suggests potentially excellent animal model for 'autoimmune' thrombocytopenia. The objective of the present study is to develop an experimental protocol that will mimic the ITP syndrome of man and subsequently offer a means of evaluating therapeutic approaches to this problem at the basic level. To achieve this objective both in vitro and in vivo approaches are to be used to examine the humoral and cellular aspects of this actively induced disease. The following studies are planned: (1) Determine the platelet type (species), dose, and route of injection that will predictably lead to acute or chronic thrombocytopenia, (2) define the humoral and cellular components of the induced thrombocytolytic state by appropriate in vitro tests, (3) passively transfer the disease by humoral or cellular means, (4) attempt therapy by splenectomy, immunosuppressive and enhancement procedures, and (5) perform immunofluorescent histopathology on selected tissues. The results of our study should provide a unique animal model that is currently lacking for the problem of immunologic thrombocytopenia in man. Establishment of the 'autoimmune' like nature of this phenomenon in the marmoset and definition of the humoral versus cellular aspects in the pathogenesis of the disease may ultimately provide some clues to the etiology and treatment of the clinical disorder.